There are various types of processes for producing reduced iron or iron alloy. There are the Waelz kiln method of heating and reducing feedstock inside a rotary kiln while tumbling with carbon and a reducing agent, the rotary hearth method of firing and reducing feedstock in a reducing rotary hearth furnace, etc. Among these, as a process with a high productivity, operation is being performed by the rotary hearth method such as shown in Japanese Unexamined Patent Publication (Kokai) No. 200054034. The rotary hearth method is a process based on heating reducing furnace of a type reducing pellets or other shaped articles of a powder feedstock charged on the hearth and moving through a heating zone, reducing zone, and ejector by rotating a disk-shaped hearth of refractories with a cutaway center at a constant speed on rails under a fixed ceiling and side walls made of refractories (hereinafter called a “rotary furnace”) and is used for the reduction of metal oxides. The diameter of the hearth of the rotary furnace is 10 to 50 meters, and the width of the hearth is 2 to 6 meters.
As the feedstock, fine ore or metal oxide dust or other metal oxides and carbon serving as a reducing agent are used. In the production of reduced iron, pellet feed ore or other particulate iron ore etc. is used. Carbon is used as the reducing agent, but an agent with a high percentage of carbon not volatilizing up to about 1100° C. at which a reduction reaction takes place, is preferable (hereinafter this carbon is called fixed carbon). For such a carbon source, fine coke or anthracite is preferred.
The powder containing the metal oxides of the feedstock is mixed with an amount of reducing agent containing carbon required for reducing the metal oxides, then is pelletized to form green pellets which are then fed in layers on the hearth of the rotary furnace. The green pellets are spread on the hearth of the rotary furnace. In a rotary furnace, a circular hearth with a cutaway center rotates under a refractory ceiling and side walls. The pellets on the hearth are moved through the several parts of the furnace and heated rapidly. They are fired for 5 minutes to 20 minutes at a high temperature of around 1300° C., whereby the metal oxides are reduced by the carbon in the pellets, and metal is produced. In the rotary furnace, since the green pellets are placed stationarily on the hearth, there is the advantage that the green pellets are resistant to crumbling in the furnace. As a result, there are the strong points that there is no problem of the powderized feedstock sticking on the refractories in the rotary furnace and the yield of pellets is high. Further, this process is being used in many cases in recent years since the productivity is high and it is possible to use an inexpensive coal-based reducing agent or powder feedstock.
Further, the rotary hearth method is also effective for treatment to reduce and remove impurities from the ironmaking dust generated from a blast furnace, converter, or electric furnace, or scale or thickener sludge generated from the rolling process. It is also used as a dust treatment process and is a process effective for recycling of metal resources. The fact that when reducing dust, the rotary hearth method is possible to remove zinc, lead, alkali metals, halogens, and other impurities is also an advantage of the rotary hearth method. This is the reason why this is a particularly effective process for recycling of dust etc. generated in the ironmaking industry.
The operation in the rotary hearth method can be summarized as follows. First, the feedstock, that is, the metal oxides such as the fine ore or the dust or sludge are mixed well with an amount of carbonaceous reducing agent required for the reduction of the oxides, then pelletized. There are several methods of pelletization. For example, green pellets of 5 to 20 mm size are produced by a pan type pelletizer or other types of pelletizer to have a moisture content of 8 to 15 wt %. These green pellets are fed onto the rotary hearth in layers and spread over the hearth. The raw pellets spread over the hearth are rapidly heated in the furnace and fired for 5 to 30 minutes at a high temperature of around 1300° C. At this time, the metal oxides are reduced by the carbon of the reducing agent mixed in with the green pellets, thus metal is produced. The amount of fixed carbon in the reducing agent is substantially determined by the amount of oxygen bonded with the metals to be reduced. The metallization ratio after reduction differs depending on the metal to be reduced, but in the case of iron, nickel, or manganese, it is at least 90% and even with the case of chromium, which is hard to be reduced, it is at least 40%.
As explained above, the rotary hearth method is an efficient, good process for reducing the metal oxides at a rotary hearth reducing furnace, in particular the dust, scale, sludge, etc. generated from the industries of refining or processing metals, to obtain reduced metal. When using dust or other by-products of the metal industry as feedstock, this is an effective means in recycling.
The fine ore used as part of the feedstock also includes chlorine or another impurities. In particular the by-products in the metal industry include machine oil, organic matter in water, chemicals with chlorine agents, resin powder, and other impurities. For example, the sludge deposited in a scale pit, which is generated in the process of rolling a steel material, contains 1 to 5 mass % of machine oil. Further, blast furnace dust contains 0.1 to 0.3 mass % of chlorine.
Further, electric furnace dust used for reduction in a reducing rotary hearth furnace also contains chlorine and oil, and in addition, it contains dioxins themselves. Most of these impurities burn or vaporize inside the rotary furnace and are discharged from the furnace. During this process, the organic matter burns and forms carbon dioxide or water vapor. If the combustion is incomplete, however, sometimes soot, carbon, unburned benzene, etc. may be contained in the combustion gas. Further, the chlorine ingredient forms chlorine gas or hydrogen chloride gas or salts such as sodium chloride, zinc chloride, etc. A large concentration of each of the materials is discharged from the furnace together with the combustion gas. These organic substances and chlorine react in the combustion gas and generate dioxins, though the amounts are small. In particular, the generation of dioxins increases when the combustion in the furnace is incomplete. When the concentration of the carbon monoxide in the combustion gas is high, the amount of dioxins generated increases due to the reaction of the chlorine and benzene or a synthesis reaction from the gas phase.
With the conventional facility configuration or method of operation, however, there was not sufficient knowledge and operation was not necessarily performed from the viewpoint of reduction of the generation of dioxins. In conventional operation, the main objective was just simply to supply heat to the reduction reaction. Further, the exhaust gas treatment initially was mainly to prevent deposition of dust in the path of the combustion gas treatment or the reclamation of waste heat. In fact, the methods of reduction of dioxins were not sufficiently reflected in the design of the facilities. In subsequent investigations, it was found that in rotary furnaces since the combustion temperature inside the furnace is higher, the amount of generation of the dioxins is smaller and the environmental load is smaller compared with other combustion processes, but dioxins cannot be totally eliminated in the combustion gas, that is, sometimes, they are present in a concentration of as much as 1 to 5 ng-TEQ/Nm3.
That is, in the operation of a reducing furnace such as a reducing furnace of the rotary hearth method according to the prior art, no good method of operation for effectively controlling the production of dioxins has yet been discovered.
Further, as explained above, a rotary furnace, Waelz kiln, or other reducing furnace generates high temperature combustion exhaust gas containing a large amount of carbon dioxide and water vapor. This combustion exhaust gas is discharged at a rate of 2000 to 3000 Nm3 per ton of feedstock. This exhaust gas contains dust generated from the inside of the furnace, passes through the exhaust gas duct, is cooled by the method of spraying water or by other methods in the exhaust gas treatment apparatus, then is cleaned of dust and emitted into the atmosphere. As explained above, the rotary furnace method is a process with a relatively large amount of generation of dust since the zinc, lead, chlorine, and other impurities are removed by vaporization during the reducing reaction of oxides.
In this way, in the operation of a rotary hearth reducing furnace, rotary kiln, or other metal reducing furnace, a large amount of combustion exhaust gas containing a large amount of dust is generated. The sensible heat held by the exhaust gas corresponds to about 30% of the total input energy. Reclamation of the heat of the combustion exhaust gas plays an important role in operation with a good heat efficiency.
However, when trying to reclaim waste heat of high temperature combustion exhaust gas, there were the problems that the dust strongly stuck to the heat transmission surfaces of the waste heat boiler or heat exchanger, or corroded the metal of these surfaces. As a result, for example in the exhaust gas treatment method in a reducing furnace, as shown in Japanese Unexamined Patent Publication (Kokai) No. 2000-169906, even in the prior art, a method of suitably controlling the temperature of the boiler or heat exchanger was adopted to prevent the deposition of dust. This method is an effective means for treatment of exhaust gas, but depending on the ingredients of the dust, there was the problem that dust deposited on the inside surfaces of the heat exchanger and the path of the exhaust gas was clogged within 2 weeks to one month or so. In particular, if lowering the melting point of the dust, the adhesion power of the dust becomes stronger and the problem becomes greater.
The dust generated from a rotary furnace etc. includes not only the carried-over substances of feedstock such as iron oxide, but also large amounts of alkali metals, zinc, lead, or other volatile metals and cationic substances such as chlorine. At the portion of the exhaust gas outlet duct of 600 to 1100° C. or so, the ingredients of the dust are present as vapor. This starts to deposit as a liquid along with a drop in the temperature of the exhaust gas. The ingredients of dust scattered as solids and the liquids form an emulsion with a high viscosity. This sticks to the path of the exhaust gas, so the path becomes narrower and the problem of easy blockage at that portion arises. That is, when installing a heat exchanger so as to reclaim waste heat, the path of the exhaust gas becomes narrower at that portion and blockage easily takes place. A liquid of an alkali metal salt is strongly cohesive and there was also the problem of metal corrosion at the portion where this emulsion stuck.
The method of the above Japanese Unexamined Patent Publication (Kokai) NO. 2000-169906 is a technique effective for the prevention of deposition of dust. In particular, when there is a large amount of alkali metal salts and zinc compounds, deposition could not be sufficiently prevented. In this way, in the prior art, sufficient attention has not been paid to the control of the ingredients of dust in the exhaust gas and the problem of clogging of a heat exchanger for reclamation of the heat of the exhaust gas has not been sufficiently solved.
Due to this, a technology has been sought which solves the above-mentioned problems and decreases the generation of dioxins in combustion exhaust gas discharged from a rotary furnace, rotary kiln, or other metal reducing furnace and efficiently reclaims sensible heat held by this exhaust gas.
All cited references are hereby incorporated herein by reference in their entireties.